Vacuum may be used in a vehicle as a motive force for adjusting a position of actuators, assisting adjustment of an actuator, and/or as a way of transferring gases from one location to another location. For example, vacuum may assist a driver applying a vehicle's brake pedal or purging fuel vapors from a fuel vapor storage canister to an engine. One way to generate vacuum is to operate an engine throttled. At lower engine loads, the engine's throttle may be partially closed to reduce engine torque. Vacuum may be generated within the engine's air intake system at a location downstream of the throttle. Thus, the engine may be a cost effective way of producing vacuum. However, smaller displacement engines tend to operate at higher intake manifold pressure as compared to larger displacement engines. Additionally, when an engine is operated at higher altitudes, it may be more difficult for the engine to produce vacuum since air pressure at higher altitudes is reduced as compared to air pressure at sea level. Consequently, an engine may operate less throttled at higher altitudes to produce a same amount of torque as at a lower altitude. Therefore, it may be more difficult to produce vacuum via the engine at higher altitudes.
One way of generating additional vacuum at higher altitudes is to reduce a load applied to the engine when the engine is operated at higher altitudes. However, it may not be possible or desirable to reduce a load applied to an engine every time vacuum is requested without degrading operation of the device providing load to the engine. As a result, the engine may produce lower vacuum than is desired, or vehicle passengers may become aggravated by degraded performance of subsystems that are unloaded from the engine for extended periods to improve vacuum generation.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method for generating vacuum, comprising: operating an air conditioning compressor bypass valve to reduce air conditioner compressor work in response to a request to reduce air conditioner compressor load for less than a predetermined amount of time.
By opening an air conditioning compressor bypass valve in response to a request for vacuum, it may be possible to provide the technical result of producing vacuum for vehicle systems in a way that is less noticeable to vehicle occupants. In particular, an air conditioning compressor bypass valve may be opened for short time periods (e.g., less than five seconds) to unload the air conditioning compressor from the engine. Thus, the air conditioning compressor may be unloaded from the engine for short periods of time so that vacuum in a vacuum reservoir may be replenished. Additionally, opening the air conditioning compressor bypass clutch allows an air conditioning compressor clutch to remain closed so that there may be less possibility of air conditioning clutch degradation.
The compressor bypass valve may be opened asynchronously with respect to instantaneous compressor piston position. For example, the compressor bypass valve may be opened when a piston of the air conditioning compressor is at any position in its cycle. However, in other examples, the compressor bypass valve (or valves) may be opened and closed in synchronism with piston position and thus continuously vary the “trapped” vapor to be compressed. For example, the compressor bypass valve may be opened during each compression stroke of the compressor piston ten degrees before top-dead-center (TDC) compression stroke. If the air conditioning compressor includes multiple pistons, the compressor bypass valve may be opened and closed synchronous with the different pistons. The compressor bypass valve opening and closing may be synchronous controlled to vary the air conditioning compressor between full compression and no (or low) compression.
The present description may provide several advantages. Namely, the approach may improve vacuum generation for a vehicle. Additionally, the approach may reduce the possibility of air conditioning clutch degradation. Further, the approach may reduce vehicle passenger discomfort by reducing air conditioning compressor load on an engine for short intervals during which a reduction in air conditioning system output may be less noticeable.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.